i've found that in most applications, a headphone out set to 2-3 out of 10 doesn't SEEM any noisier than a line level out and is approximately the same volume-in fact, when using a headphone out into something analog, i may kick it up just a bit more and mess with it until i hear what i like (for example, with a portable cd player in a car). i know that amplification is best saved for the part of the signal chain nearest the speakers if possible, but sometimes, it's worth settling for .02% distortion to make an otherwise kinda quiet line level device keep up a little better to the levels you want. just a thought.
on the trs plug issue, the tip should still be positive hot and the sleeve should still be ground, so i wouldn't worry about the plugging and just try it-you'll lose a balanced signal, but it shouldn't be any different than jsut running TS.